The ongoing standalone LTE-U forum and future 3GPP Rel-14 work item on Uplink Licensed-Assisted Access (LAA) intends to allow LTE UEs to transmit on the uplink in the unlicensed 5 GHz or license-shared 3.5 GHz radio spectrum. For the case of standalone LTE-U, respectively the MulteFire (MF) project, the initial random access and subsequent UL transmissions take place entirely on the unlicensed spectrum. Regulatory requirements may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method needs to be applied for channel sensing. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Therefore, the initial random access (RA) procedure for standalone LTE-U should involve as few transmissions as possible and also have low latency, such that the number of LBT operations can be minimized and the RA procedure can then be completed as quickly as possible.
Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known under its marketing brand as “Wi-Fi.”